gsoc26: Format Conversion Layer (Layer 2) + CLI refactor (#59, #61)

[DhanashreePetare]

Pull Request

Description

Implements Layer 2 (Format Conversion) for the Databus Python Client download pipeline, bringing it to feature parity with the Java client as described in Frey et al. Users can now convert between RDF serialization formats and tabular formats on-the-fly during download using the new --format flag. Also refactors the compression CLI (Issue #61) by replacing --convert-to / --convert-from with a single --compression flag.

Related Issues
Issue #59 (Format and Mapping Conversion Layer — Layer 2)
Issue #61 (Refactor CLI compression)

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• This change requires a documentation update
• Housekeeping

Checklist:

• My code follows the ruff code style of this project.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
  • poetry run pytest - all tests passed
  • poetry run ruff check - no linting errors

What was added:

databusclient/filehandling/format.py — Layer 2 with TripleHandler, QuadHandler, TSDHandler classes using rdflib.Graph, rdflib.Dataset, and list[list[str]] as intermediate representations respectively. Each handler exposes read(), write(), and convert().

What was changed:

• --format flag replaces --convert-format, short aliases added (nt, ttl, rdf, xml, nq, jsonld)
• --compression replaces --convert-to / --convert-from (source auto-detected from file extension)
• Download pipeline: decompress once → convert → recompress once (was double decompress/recompress)
• Original file deleted after successful conversion
• Safe temp file handling via tempfile.NamedTemporaryFile

Tests:

• 9 format round trip tests in tests/test_format_round_trips.py (one per format, IR captured before conversion)

Closes #59
Closes #61

Summary by CodeRabbit

• New Features

  • Added RDF format conversion support for triples and quads across multiple formats
  • Added tabular format conversion support (CSV/TSV)
  • Updated download CLI with new --compression and --format options
  • Added automatic source compression detection during downloads

• Documentation

  • Updated README with revised CLI documentation and usage examples

• Tests

  • Added comprehensive format round-trip conversion tests
  • Added test data fixtures for multiple RDF formats

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